Vertically aligned nematic mode liquid crystal display having large tilt angles and high contrast

ABSTRACT

A reflective liquid crystal on silicon (LCOS) display comprises a transparent substrate, a reflective substrate, and liquid crystal fluid between the substrates. The LCOS display further comprises a matrix of pixels, arranged in a plurality of rows and columns, wherein an intersection of a row and a column defines a position of a pixel in the matrix. The LCOS display has tilt angles sufficient to overcome disclinations due to fringe fields, and, at the same time, achieves high contrast. The surface azimuthal direction of the molecules of the liquid crystal fluid is either substantially parallel or perpendicular to the direction of polarization of incoming incident linearly polarized light. Light leakage is minimal because the effective birefringence as seen by the incoming incident linearly polarized light is substantially zero and does not depend on the pretilt of the molecules of the liquid crystal fluid. Between the transparent substrate and the reflective substrate, the twist of the molecules of the liquid crystal fluid may vary from about 0 degrees to about 90 degrees when in the “OFF” state.

RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/450,370, entitled “Method to Eliminate the DisclinationDefects Due to Fringe Fields in Vertically Aligned Nematic Reflective LCDisplays Without Hurting the Display Contrast” by Hemasiri Vithana,filed Feb. 26, 2003, and is incorporated herein by reference for allpurposes.

FIELD OF THE INVENTION

The present invention relates generally to liquid crystal displays(LCDs), and more particularly to reflective liquid crystal on silicon(LCOS) displays.

BACKGROUND OF THE FIELD OF TECHNOLOGY

Liquid crystal display technology has reduced the size of displays fromfull screen sizes to minidisplays of less than 1.3 inches diagonalmeasurement, to microdisplays that require a magnification system.Microdisplays may be manufactured using semiconductor integrated circuit(IC) dynamic random access memory (DRAM) process technologies, e.g.,liquid crystal on silicon (LCOS). The LCOS microdisplays consist of asilicon substrate backplane with a reflective surface, a cover glass andan intervening liquid crystal layer. The LCOS microdisplays are arrangedas a matrix of pixels arranged in a plurality of rows and columns,wherein an intersection of a row and a column defines a position of apixel in the matrix. To incident light, each pixel is a liquid crystalcell above a reflecting mirror. By changing the molecular orientation ofthe liquid crystal in the layer, characterized by a tilt angle and atwist angle of the liquid crystal director at any point in the layer,the incident light can be made to change its polarization. The siliconbackplane is an array of pixels, typically 7 to 20 micrometers (μm) inpitch. Each pixel has a mirrored surface that occupies most of the pixelarea. The mirrored surface is also an electrical conductor that forms apixel capacitor with the liquid crystal display cover glass electrodewhich is a transparent conductive coating on the inside face (liquidcrystal side) of the cover glass. This transparent conductive coating istypically Indium Tin Oxide (ITO). As each pixel capacitor is charged toa certain voltage value, the liquid crystal fluid between the plates ofthe pixel capacitors changes its molecular orientation which affects thepolarization state of the light incident to the pixels (reflections fromthe pixel mirrors).

The reflective LCOS microdisplays have a high aperture ratio, andtherefore can provide greater brightness than transmissive liquidcrystal displays. Major applications of these LCOS microdisplays are inhome theater applications, e.g., projectors, and front and rearprojection televisions (large screen). For these applications, highcontrast is very important. High contrast depends upon the liquidcrystal optical mode being used in the liquid crystal display.Typically, a Vertically Aligned Nematic (VAN) mode is one of the opticalmodes that can achieve a very high contrast and many liquid crystaldisplay manufacturers are beginning to use this particular optical modein their displays.

The pretilt angle is defined as the tilt angle of the liquid crystaldirector at the boundary surface. In VAN mode liquid crystal displays,the pretilt angle is small, so the orientation of the molecules of theliquid crystal fluid are nearly perpendicular to the substrate surfaceswhen there is no electric field applied across the display. Therefore,incoming linearly polarized light, perpendicular to the displaysubstrates, sees a small birefringence as it passes through the layer.Hence this normally incident linearly polarized light experiences littlephase retardation when going through the liquid crystal fluid, includingbeing reflected back from the bottom reflective substrate of thedisplay. This provides a very dark “OFF” state when using crossedpolarizers (e.g., polarizing beam splitter—PBS), thus very high contrastis achieved. Upon application of an electric field across the liquidcrystal fluid, the molecules in the bulk of the liquid crystal fluidorient themselves toward a direction defined by alignment layers on thesubstrate surfaces, thereby increasing the retardation of the layer ofthe liquid crystal fluid. Therefore, linearly polarized incident lightstarts to experience a phase retardation when going into the liquidcrystal fluid and then being reflected back from the bottom reflectivesubstrate of the display. As a result of this, the polarization state ofthe out-going light (reflected light) will be elliptical and some lightstarts to pass through the crossed polarizers. Increasing the electricfield increases this effect until the brightest state is achieved.

In a typical VAN mode, the orientations of the molecules of the liquidcrystal fluid at the substrate surfaces are defined by the alignmentlayers on each of the substrate surfaces. This orientation is describedby a pretilt angle and a surface azimuthal direction, which is parallelto the projection of the liquid crystal director onto the plane of thesubstrate. The azimuthal direction of the molecules of the liquidcrystal fluid proximate to the top alignment layer is opposite to theazimuthal direction of the molecules of the liquid crystal fluidproximate to the bottom alignment layer, i.e., anti-parallel. Theazimuthal directions defined by the alignment layers are at a 45 degreeangle with the direction of polarization of the incoming linearlypolarized incident light, as shown in Figures la and lb. Usually thepretilt angle of the molecules in a VAN mode display needs to be keptsmall, e.g., less than 4 degrees, to achieve a very dark “OFF” state,hence the high contrast. Although this pretilt angle is large enough toprevent reverse tilt domains in the display, it is not possible toovercome the defects that occur due to fringe fields between neighboringpixels. Fringe fields become very significant when the pixel sizebecomes small as is typical in LCOS microdisplays. For example, the sizeof an LCOS microdisplay may measure one inch diagonally and have a pixelsize of approximately 12 μm×12 μm. When high resolution is required,e.g., digital cinema applications, pixel size may be further reduced toapproximately 9 μm×9 μm or even smaller. In such situations fringefields are quite pronounced and the liquid crystals do not align alongthe direction defined by the tilt direction of the alignment layers.Ultimately this will create defects at the pixel boundaries, usuallyknown as disclinations. This is quite apparent when one pixel is in an“ON” state and an adjacent pixel is in an “OFF” state, wherein thefringe fields are very strong.

To overcome the above problem, it is necessary to increase the pretiltangle generated by the alignment layers on the substrate surfaces.Experimentally it has been determined that the pretilt angle has to beat least 8 degrees to overcome the fringe field effects. However, thedark state of a VAN mode liquid crystal display with a pretilt angle ofthis magnitude has a significant amount of light leakage through thecrossed polarizers and the light contrast it can achieve is not thathigh. Therefore, the inherent property of VAN displays, the very dark“OFF” state, cannot be fully achieved. This is due to the non-zerobirefringence seen by the linearly polarized incident light due to thehigh pretilt angle of the liquid crystal fluid. Heretofore, it has beennecessary to use other methods such as attaching external retarders tostop this light leakage. Generally, this is the current method used byall the VAN liquid crystal display manufactures to solve the aboveproblem.

SUMMARY OF THE INVENTION

The present invention overcomes the above-identified problems as well asother shortcomings and deficiencies of existing technologies byproviding a system, method and apparatus having pretilt anglessufficient to overcome disclinations due to fringe fields, and, at thesame time, achieving high contrast.

In the typical VAN optical mode, the surface azimuthal direction is 45degrees to the direction of polarization of incoming linearly polarizedincident light. Therefore, there is an effective birefringence as seenby the incoming incident light and it increases with increasing pretiltangle, hence the amount of light leakage.

If the display shown in FIG. 1 a is rotated by 45 degrees with respectto the polarization direction of the incident linearly polarized light,i.e., the surface azimuthal direction of the molecules of the liquidcrystal fluid is either parallel or perpendicular to the direction ofpolarization of incoming incident linearly polarized light, then thelight leakage is minimum because the effective birefringence as seen bythe incoming incident linearly polarized light is zero and does notdepend on the pretilt of the molecules of the liquid crystal fluid.However, this configuration cannot be applied to practical applicationsbecause the “ON” state is not bright due to the same reason explainedabove. But this feature is advantageously being used in the presentinvention, i.e., surface azimuthal directions generated by the top andbottom substrates are set substantially perpendicular to each other.Also at the same time, the surface azimuthal direction generated by onesubstrate is perpendicular/parallel to the direction of polarization ofincoming linearly polarized light while the surface azimuthal directionfrom the other substrate is parallel/perpendicular. Essentially this isa 90 degree twisted structure in the “OFF” state. Because the majorityof the molecules of the liquid crystal layer do not have their azimuthaldirections oriented at 45 degrees to the direction of polarization ofincoming incident linearly polarized light, the effective birefringenceas seen by the incoming incident linearly polarized light is minimal ascompared to a VAN structure where all molecules have the same tilt angleand their surface azimuthal directions are all oriented at 45 degreeswith respect to the polarization direction of the incoming linearlypolarized light. Therefore, in the present invention, the light leakageis very small even though the pretilt angle is large enough to removethe disclinations due to fringe fields.

An important technical feature of the optical mode of the presentinvention takes place when in the “ON” state. In the “ON” state, theinvention behaves differently from a conventional 90 degree twistednematic (TN) mode, and the invention gives a very good bright state asis desired in liquid crystal display applications using a PBS. In aconventional 90 degree TN mode the linear polarized light is “guided” bythe twisted structure, both in going in and coming out of the layer,which would give a dark state with a PBS. According to the presentinvention, the bright “ON” state is achieved without having to addchiral dopant in the liquid crystal fluid of the display.

Although the surface azimuthal directions of the molecules of the liquidcrystal fluid at the bottom and top substrates produce a 90 degreetwist, this optical mode does not behave as does a conventional 90degree twisted nematic mode in the “ON” state. On the other hand, thepresent invention does not behave as a VAN mode display in the “OFF”state. The present invention produces a much darker state than that of aregular VAN mode display with the same pretilt angle. Therefore, thepresent invention is neither a VAN nor a conventional 90 degree twistednematic mode display.

A gray scale, e.g., a light intensity through the polarizer that isintermediate to the “ON” and “OFF” intensities may be obtained bycontrolling the voltage that is applied across the liquid crystal layer.In a crossed-polarizer (or PBS) configuration, increasing the voltageincreases the light passing to the output up to a certain optimumbright-state voltage. The value of this optimum bright state voltagedepends upon the liquid crystal material parameters, the cell gap, thepretilt and the light wavelength range of interest. This voltage can bedetermined experimentally. In addition, perceived gray scale can becontrolled by controlling the time that the liquid crystal display is inthe “ON” state and the time in the “OFF” state. In addition, color maybe generated with methods known in the art such as color filters, use ofthree displays each for one color in a three-panel system, or with onemicrodisplay in a field sequential color (FSC) system.

The thickness (d) of the liquid crystal fluid (distance between theinside faces of the top and bottom substrates) may be, for example,about 3.5 μm +/−0.2 μm. The birefringence (Δn) may be, for example,about 0.0830 at 45° C. Liquid crystal fluids that are being used in thepresent invention are nematic and have negative dielectric anisotropyΔε(=ε_(∥)−ε_(⊥)0), where ε_(∥) and ε_(⊥) are the parallel andperpendicular (to the liquid crystal molecule) components of thedielectric constant of the liquid crystal material. It is contemplatedand within the scope of the present invention that any combination ofthickness (d) and birefringence (Δn) may be used so long as thecondition: Δn·d>λ/4 is satisfied, where λ is wavelength of lightincident on the display.

Any liquid crystal fluids developed for VAN displays may be used in thepresent invention. According to the present invention, it is notnecessary to introduce a chiral dopant into the liquid crystal fluid.Typical liquid crystal fluids are for example but not limited to:MLC-6608, MLC-6609 and MLC-6610 manufactured by Merck. A physicalproperty of liquid crystal materials used for VAN displays is negativedielectric anisotropy, i.e., the perpendicular component of thedielectric constant is larger than that of the parallel component.Therefore, with an applied electric field, the molecules of the liquidcrystal fluid will arrange themselves perpendicular to the direction ofthe electric field. For example, the dielectric anisotropy may rangefrom about Δε=−3.1 to −4.2. Birefringence may range from about 0.0777 toabout 0.0996, and the nematic to isotropic phase transition temperaturemay be above 80° C.

A technical advantage of the present invention is that with this opticalmode it is possible to have a much darker “OFF” state, hence highcontrast, even with a relatively high pretilt angle compared to the VANmode display with the same pretilt angle. Because of the high pretiltangle substantially no disclination defects will occur due to the fringefields across the neighboring pixels at the pixel boundaries. Anothertechnical advantage is that external retarders are not necessary toblock the light leakage because of the very good dark state of thepresent invention.

In projection applications with McNeil type polarizing beam splitters,there is a system retarder, usually a quarter wave plate, for each andevery color channel (RGB) to compensate the skew rays. When regular VANmode displays are accommodated in such applications, then this systemretarder can also be used to stop the light leakage. Essentially it isgoing to be a compromise state between the skew ray compensation andlight leakage. However, this does not work well for the BLUE channelgiving rise to a reasonable amount of light leakage. Therefore, contrastof the BLUE channel is generally lower than the other two channels (REDand GREEN). In fact the proper retarders to stop the light leakage arethe ones with low retardation values of around 50 nanometers or less asshown by experiments. Such retarders with good uniformity are difficultto find and not easily available commercially. According to the presentinvention, this will not be a problem because of the very dark “OFF”state across the visible spectral region and the system retarders can beused solely to compensate for the skew rays. Also the BLUE channel willnot suffer with low contrast either.

Some optical architectures do not require skew ray compensation. Anexample of this type of architecture are those where a wire-gridpolarizing beamsplitter is used to separate input and output beam paths.Such wire-grid beamsplitters are manufactured by Moxtek Inc., of Orem,Utah. Therefore, for such applications the present invention isadvantageous since the extra cost of attaching external retarders can beeliminated.

The present invention is directed to a reflective liquid crystaldisplay, comprising: a first substrate that is substantiallytransparent; a second substrate that is substantially reflective andsubstantially parallel with said first substrate; and a liquid crystalfluid having a birefringence (Δn) and a negative dielectric anisotropy,wherein said liquid crystal fluid is between said first and secondsubstrates; said first substrate having a first liquid crystal alignmentlayer proximate to said liquid crystal fluid, wherein molecules of saidliquid crystal fluid that are proximate to the first liquid crystalalignment layer have a first pretilt angle from about 2 degrees to about15 degrees and a first azimuthal direction; said second substrate havinga second liquid crystal alignment layer proximate to said liquid crystalfluid, wherein molecules of said liquid crystal fluid that are proximateto the second liquid crystal alignment layer have a second pretilt anglefrom about 2 degrees to about 15 degrees and a second azimuthaldirection, the second azimuthal direction being substantiallyperpendicular to the first azimuthal direction.

The polarization direction of the linearly polarized incident light maybe approximately parallel with the first surface azimuthal direction.The polarization direction of the linearly polarized incident light maybe approximately perpendicular with the first surface azimuthaldirection.

Shades of gray may be produced by varying the electric field betweensaid first and second substrates from substantially no electric field toan electric field having an optimum value.

The present invention is also directed to a reflective liquid crystaldisplay, comprising: a first substrate that is substantiallytransparent; a second substrate that is substantially reflective andsubstantially parallel with said first substrate; and a liquid crystalfluid having a birefringence (Δn) and a negative dielectric anisotropy,wherein said liquid crystal fluid is between said first and secondsubstrates; said first substrate having a first liquid crystal alignmentlayer proximate to said liquid crystal fluid, wherein molecules of saidliquid crystal fluid that are proximate to the first liquid crystalalignment layer have a first pretilt angle from about 2 degrees to about15 degrees and a first azimuthal direction; said second substrate havinga second liquid crystal alignment layer proximate to said liquid crystalfluid, wherein molecules of said liquid crystal fluid that are proximateto the second liquid crystal alignment have a second pretilt angle fromabout 2 degrees to about 15 degrees and a second azimuthal direction,the second azimuthal direction being substantially perpendicular to thefirst azimuthal direction; wherein: when an electric field is appliedbetween said first and second substrates, a substantial number of themolecules of said liquid crystal fluid increase their tilt angles, andwhen substantially no electric field is applied between said first andsecond substrates, a substantial number of the molecules of said liquidcrystal fluid have their azimuthal direction substantially perpendicularto said first and second substrates; whereby when the electric field isapplied between said first and second substrates, said liquid crystalfluid changes linear polarized incident light at said first substrate toapproximately circularly polarized incident light when at said secondsubstrate, wherein said second substrate reflects back the approximatelycircularly polarized incident light as approximately circularlypolarized light of opposite handedness, wherein said liquid crystalfluid changes the approximately circularly polarized reflected light toapproximately linear polarized reflected light when at said firstsubstrate such that the polarization directions of the linearlypolarized incident light and the linearly polarized reflected light atsaid first substrate are approximately perpendicular, and whereby whensubstantially no electric field is applied between said first and secondsubstrates, said liquid crystal fluid does not substantially change thepolarization of light passing through the liquid crystal fluid and thepolarization directions of the linearly polarized incident light and thelinearly polarized reflected light at said first substrate areapproximately parallel.

The electric field may be applied between said first and secondsubstrates for a plurality of first times and substantially no electricfield may be applied for a plurality of second times, wherein the firstand second times may be varied to produce shades of gray. The first andsecond times may also be varied to produce field sequential colors.

The present invention is directed to a method for a reflective liquidcrystal display, said method comprising the steps of: providing a firstsubstrate that is substantially transparent; providing a secondsubstrate that is substantially reflective and substantially parallelwith said first substrate; and providing a liquid crystal fluid having abirefringence (Δn) and a negative dielectric anisotropy between saidfirst and second substrates; providing a first liquid crystal alignmentlayer on said first substrate, said first liquid crystal alignment layerbeing proximate to said liquid crystal fluid, wherein molecules of saidliquid crystal fluid that are proximate to the first liquid crystalalignment layer have a first pretilt angle from about 2 degrees to about15 degrees and a first azimuthal direction; providing a second liquidcrystal alignment layer on said second substrate, said second liquidcrystal alignment layer being proximate to said liquid crystal fluid,wherein molecules of said liquid crystal fluid that are proximate to thesecond liquid crystal alignment layer have a second pretilt angle fromabout 2 degrees to about 15 degrees and a second azimuthal direction,the second azimuthal direction being substantially perpendicular to thefirst tilt orientation direction; such that when substantially noelectric field is applied between said first and second substrates, saidliquid crystal fluid does not change the polarization state of lightpassing therethrough and the polarization directions of the linearlypolarized incident light and the linearly polarized reflected light atsaid first substrate are substantially parallel, when applying anelectric field of an optimum value between said first and secondsubstrates, said liquid crystal fluid changes linear polarized incidentlight at said first substrate to approximately circularly polarizedincident light when at said second substrate, wherein said secondsubstrate reflects back the approximately circularly polarized incidentlight as approximately circularly polarized light of oppositehandedness, wherein said liquid crystal fluid changes the approximatelycircularly polarized reflected light to approximately linear polarizedreflected light when at said first substrate such that the polarizationdirections of the linearly polarized incident light and the linearlypolarized reflected light at said first substrate are approximatelyperpendicular, and when applying an electric field less than the optimumvalue between said first and second substrates, said liquid crystalfluid changes the polarization state of the incident linearly polarizedlight to elliptically polarized light upon passing through said liquidcrystal fluid and being reflected back from said second substrate.

Varying the electric field between said first and second substrates maybe used to produce shades of gray by causing the polarization state ofthe incident light at said second substrate to vary from approximatelylinear polarization to elliptical polarization, and when there issubstantially no electric field the polarization of the incident lightat said second substrate is approximately linear.

The present invention is also directed to a method for a reflectiveliquid crystal display, said method comprising the steps of: providing afirst substrate that is substantially transparent; providing a secondsubstrate that is substantially reflective and substantially parallelwith said first substrate; and providing a liquid crystal fluid having abirefringence (Δn) and a negative dielectric anisotropy, wherein saidliquid crystal fluid is between said first and second substrates;providing a first liquid crystal alignment layer on said firstsubstrate, said first liquid crystal alignment layer being proximate tosaid liquid crystal fluid, wherein molecules of said liquid crystalfluid that are proximate to the first liquid crystal alignment layerhave a first pretilt angle from about 2 degrees to about 15 degrees anda first azimuthal direction; providing a second liquid crystal alignmentlayer on said second substrate, said second liquid crystal alignmentlayer being proximate to said liquid crystal fluid, wherein molecules ofsaid liquid crystal fluid that are proximate to the second liquidcrystal alignment layer have a second pretilt angle from about 2 degreesto about 15 degrees and a second azimuthal direction, the secondazimuthal direction being substantially perpendicular to the firstazimuthal direction; wherein: when substantially no electric field isapplied between said first and second substrates, a substantial numberof the molecules of said liquid crystal fluid have their orientationsubstantially perpendicular to said first and second substrates; andwhen applying an electric field between said first and secondsubstrates, a substantial number of the molecules of said liquid crystalfluid change tilt towards parallel to the substrates; whereby whenapplying the electric field between said first and second substrates,said liquid crystal fluid changes linear polarized incident light atsaid first substrate to approximately circularly polarized incidentlight when at said second substrate, wherein said second substratereflects back the approximately circularly polarized incident light asapproximately circularly polarized light of opposite handedness, whereinsaid liquid crystal fluid changes the approximately circularly polarizedreflected light to approximately linear polarized reflected light whenat said first substrate such that the polarization directions of thelinearly polarized incident light and the linearly polarized reflectedlight at said first substrate are approximately perpendicular, andwhereby when no electric field is applied between said first and secondsubstrates, said liquid crystal fluid does not substantially change thepolarization state of light passing through said liquid crystal fluidsuch that the polarization directions of the linearly polarized incidentlight and the linearly polarized reflected light at said first substrateare approximately parallel.

The present invention is also directed to a reflective liquid crystaldisplay assembly comprising: a first substantially transparentsubstrate; a second substantially reflective substrate locatedsubstantially parallel to said first substrate; a liquid crystal fluidhaving a birefringence (Δn) and a negative dielectric anisotropy,wherein said liquid crystal fluid is between said first and secondsubstrates; first and second liquid crystal alignment layers on saidfirst and second substrates, respectively, wherein molecules of saidfluid proximate the first and second liquid crystal alignment layershave finite pretilt angles and are oriented in first and secondazimuthal directions , respectively; wherein the assembly is configuredsuch that (i) when substantially no electric field is applied betweenthe substrates, a substantial number of fluid molecules are orientedsubstantially perpendicular to said substrates, (ii) when an electricalfield of optimum value is applied between the substrates, tilt angles ofa substantial number of fluid molecules increase, and (iii) when a lessthan optimum electric field is applied between said substrates, asubstantial number of fluid molecules are oriented at intermediate tiltangles.

Other technical advantages of the present disclosure will be readilyapparent to one skilled in the art from the following figures,descriptions, and claims. Various embodiments of the invention obtainonly a subset of the advantages set forth. No one advantage is criticalto the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present disclosure and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, wherein:

FIGS. 1 a and 1 b are schematic representations of a prior art VAN modeliquid crystal display in the “OFF” and “ON” states, respectively;

FIG. 2 is a schematic elevational view of a portion of the liquidcrystal display showing the azimuthal direction and pretilt angles of anexemplary molecule in a liquid crystal fluid;

FIGS. 3 a and 3 b are schematic representations of a liquid crystaldisplay in the “OFF” and “ON” states, respectively, according to thepresent invention; and

FIG. 4 is a collection of graphical representations of the tilt anglesand azimuthal angles of liquid crystal fluid molecules as a function ofthe molecules' locations between the substrates in “OFF,” and “ON”states, according to the present invention.

While the present invention is susceptible to various modifications andalternative forms, specific exemplary embodiments thereof have beenshown by way of example in the drawings and are herein described indetail. It should be understood, however, that the description herein ofspecific embodiments is not intended to limit the invention to theparticular forms disclosed, but on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention is directed to a reflective liquid crystalmicrodisplay comprising a matrix of pixels of a liquid crystal fluidhaving light modifying properties controlled by voltage values stored incapacitors comprising the areas representing the pixels in the matrix ofpixels of the liquid crystal microdisplay. The molecules of the liquidcrystal fluid have a surface azimuthal direction that is eitherapproximately parallel or perpendicular to the direction of polarizationof incoming incident linearly polarized light.

According to the present invention, when in the “OFF” state (no electricfield across the pixel capacitors), the molecules of the liquid crystalfluid have pretilt angles (measured from the perpendicular directions ofthe faces of the substrates) just sufficient to minimize disclinationsdue to fringe fields. The azimuthal direction of the molecules of theliquid crystal fluid at the transparent substrate is at approximately 90degrees from the azimuthal direction of the molecules of the liquidcrystal fluid at the reflective substrate. This configuration results ina very dark state when in the “OFF” state, resulting in high contrast.

Application of a voltage to the liquid crystal layer changes the tilt ofthe liquid crystal molecules in the bulk of the layer in a directiontowards parallel to the substrates, due to the negative dielectricanisotropy of the material (see FIG. 4).

Referring now to the drawings, the details of exemplary embodiments ofthe invention are schematically illustrated. Like elements in thedrawings will be represented by like numbers, and similar elements willbe represented by like numbers with a different lower case lettersuffix.

Referring to FIGS. 1 a and 1 b, depicted are schematic representationsof a prior art VAN mode liquid crystal display in the “OFF” and “ON”states, respectively. The azimuthal direction of the liquid crystalmolecules defined by the alignment layers on the substrate surfaces 102,104, are anti-parallel to each other, and are at a 45 degree angle withthe direction of polarization of incoming incident linearly polarizedlight. In a VAN mode liquid crystal display, the pretilt angle θ needsto be kept small, e.g., less than 4 degrees, to achieve a very dark“OFF” state, hence high contrast.

Referring now to FIG. 2, depicted is a schematic elevational view of aportion of a liquid crystal display showing the azimuthal directionangle (Φ) and tilt angle (θ) of an exemplary molecule in a liquidcrystal fluid. A glass (transparent) substrate 202 and a reflective(mirror) substrate 204 are parallel and have liquid crystal fluid,generally represented by the numeral 206, therebetween. The distance(thickness of liquid crystal fluid) between these parallel substrates isgenerally represented by “d.” The distance (thickness), d, preferablymay be about 3.5 μm +/−0.2 μm. The liquid crystal fluid 206 preferablymay have a birefringence (Δn) of about 0.0830 at 45° C. Liquid crystalfluids that are being used in the present invention are nematic and havenegative dielectric anisotropy Δε(=ε_(∥)−ε_(⊥)<0), where ε_(∥) and ε_(⊥)are the parallel and perpendicular (to the nematic director) componentsof the dielectric constant of the liquid crystal fluid. It iscontemplated and within the scope of the present invention that anycombination of distance (thickness) (d) and birefringence (Δn) may beused with the condition: Δn·d>λ/4 is approximately satisfied for anefficient bright state at a convenient voltage, (where λ is thewavelength of light incident on the display). For example, with a cellgap of approximately 3.5 μm, using MLC-6608, and a pretilt ofapproximately 8 degrees from surface-normal, the optimum bright statevoltage is approximately 4.0V for Red (640 nm), 3.5V for Green (540 nm)and 3.15V for Blue (470 nm).

A single molecule 206 a of the liquid crystal fluid 206 is shown forexemplary purposes. The tilt angle θ is measured from the Z-axis whichis perpendicular to the substrates 202 and 204. The azimuthal angle Φ ismeasured from the X-axis in the XY plane and is the angle between the Xaxis and the projection of the molecules of the liquid crystal fluid 206on the XY plane. According to the present invention, polarizationdirection of the linearly polarized incident light at the glasssubstrate will be either approximately parallel or perpendicular to theazimuthal direction generated at the glass substrate 202.

Referring to FIG. 3 a, depicted is a schematic representation of aliquid crystal display in the “OFF” state, according to the presentinvention. The molecules of the liquid crystal fluid 206 are depicted inthe “OFF” state wherein the pretilt angle θ (defined above) is onlylarge enough to remove disclinations due to fringe fields. Preferablythe pretilt angle θ may be from about 2 degrees to about 15 degrees.Most preferably the pretilt angle θ may be from about 5 degrees to about15 degrees. The polarization state of the incident light 308 will not beaffected substantially by the molecules of the liquid crystal fluid 206and will be reflected back from the substrate 204 as substantiallylinearly polarized light with the polarization direction substantiallyparallel with that of the incident linearly polarized light.

Referring to FIG. 3 b, depicted is a schematic representation of aliquid crystal display in the “ON” state, according to the presentinvention. The configuration of the liquid crystal fluid 206, formedunder optimum voltage drive, will cause the linearly polarized incidentlight entering the substrate 202 to become approximately circularlypolarized light at the substrate 204. The approximately circularlypolarized incident light at the substrate 204 will be reflected back asapproximately circularly polarized light, but with opposite handedness,and when it reaches the substrate 202, this reflected light will beapproximately linearly polarized with a polarization directionapproximately perpendicular to that of the linearly polarized incidentlight.

Referring now to FIG. 4, depicted are graphical representations of thetilt angles and azimuthal angles of molecules of the liquid crystalfluid 206 as a function of the location of these molecules locatedbetween the substrates in “OFF” and “ON” states, according to thepresent invention. Tilt angle θ is depicted on the left vertical axisfrom 0 to 90 degrees, and the azimuthal angle Φ is depicted on the rightvertical axis from 0 to 90 degrees. The locations of the molecules ofthe liquid crystal fluid 206 between the substrates 202 and 204 aredepicted on the horizontal axis from 0 to d. Curve 402 depicts the tiltangle θ of the molecules of the liquid crystal fluid 206 when they arein the “OFF” state. Curve 404 depicts the azimuthal angle Φ of themolecules of the liquid crystal fluid 206 when they in the “OFF” state.Curve 410 depicts the tilt angle θ of the molecules of the liquidcrystal fluid 206 when they are in the “ON” state. Curve 412 depicts theazimuthal angle Φ of the molecules of the liquid crystal fluid 206 whenthey are in the “ON” state. Corresponding tilt curves for anintermediate gray shade would fall between curves 402 and 410 andazimuthal curves would fall between curves 404 and 412.

The invention, therefore, is well adapted to carry out the objects andattain the ends and advantages mentioned, as well as others inherenttherein. While the invention has been depicted, described, and isdefined by reference to exemplary embodiments of the invention, suchreferences do not imply a limitation on the invention, and no suchlimitation is to be inferred. The invention is capable of considerablemodification, alternation, and equivalents in form and function, as willoccur to those having ordinarily skills in the pertinent arts and havingthe benefit of this disclosure. The depicted and described embodimentsof the invention are exemplary only, and are not exhaustive of the scopeof the invention. Consequently, the invention is intended to be limitedonly by the spirit and scope of the appended claims, giving fullcognizance to equivalents in all respects.

1. A reflective liquid crystal display, comprising: a first substratethat is substantially transparent; a second substrate that issubstantially reflective and substantially parallel with said firstsubstrate; and a liquid crystal fluid having a birefringence (Δn) and anegative dielectric anisotropy, wherein said liquid crystal fluid isbetween said first and second substrates; said first substrate having afirst liquid crystal alignment layer proximate to said liquid crystalfluid, wherein molecules of said liquid crystal fluid that are proximateto the first liquid crystal alignment layer have a first pretilt anglefrom about 2 degrees to about 15 degrees and a first azimuthaldirection; said second substrate having a second liquid crystalalignment layer proximate to said liquid crystal fluid, whereinmolecules of said liquid crystal fluid that are proximate to the secondliquid crystal alignment layer have a second pretilt angle from about 2degrees to about 15 degrees and a second azimuthal direction, the secondazimuthal direction being substantially perpendicular to the firstazimuthal direction.
 2. The reflective liquid crystal display of claim1, wherein linearly polarized incident light has a polarizationdirection approximately parallel to the first azimuthal direction. 3.The reflective liquid crystal display of claim 1, wherein linearlypolarized incident light has a polarization direction approximatelyperpendicular to the first azimuthal direction.
 4. The reflective liquidcrystal display of claim 1, wherein shades of gray are produced byvarying an electric field between said first and second substrates fromsubstantially no electric field to an electric field having an optimumvalue.
 5. The reflective liquid crystal display of claim 1, wherein adistance (d) between inside faces of said first and second substrates isabout 3.5 micrometers.
 6. The reflective liquid crystal display of claim1, wherein a distance (d) between inside faces of said first and secondsubstrates is from about 3.3 micrometers to about 3.7 micrometers. 7.The reflective liquid crystal display of claim 1, wherein thebirefringence (Δn) is about 0.0830 at about 45 degrees Celsius.
 8. Thereflective liquid crystal display of claim 1, wherein the birefringence(Δn) is from about 0.0777 to about 0.0996.
 9. The reflective liquidcrystal display of claim 1, wherein the birefringence (Δn) times d isgreater than λ/4 when an electric field having an optimum value isapplied between said first and second substrates, where d is thedistance between inside faces of said first and second substrates and λis the wavelength of light.
 10. The reflective liquid crystal display ofclaim 1, wherein the molecules of said liquid crystal fluid have a tiltangle (θ) of from about 5 degrees to about 15 degrees when substantiallyno electric field is applied between said first and second substrates.11. The reflective liquid crystal display of claim 1, wherein themolecules of said liquid crystal fluid have a tilt angle (θ) of fromabout 2 degrees to about 15 degrees when substantially no electric fieldis applied between said first and second substrates.
 12. The reflectiveliquid crystal display of claim 1, wherein an azimuthal angle (Φ) of themolecules of said liquid crystal fluid varies from about 0 degrees atsaid first substrate to about 90 degrees at said second substrate whensubstantially no electric field is applied between said first and secondsubstrates.
 13. The reflective liquid crystal display of claim 1,wherein an azimuthal angle (Φ) of the molecules of said liquid crystalfluid varies from about 0 degrees at said second substrate to about 90degrees at said first substrate when substantially no electric field isapplied between said first and second substrates.
 14. A reflectiveliquid crystal display, comprising: a first substrate that issubstantially transparent; a second substrate that is substantiallyreflective and substantially parallel with said first substrate; and aliquid crystal fluid having a birefringence (Δn) and a negativedielectric anisotropy, wherein said liquid crystal fluid is between saidfirst and second substrates; said first substrate having a first liquidcrystal alignment layer proximate to said liquid crystal fluid, whereinmolecules of said liquid crystal fluid that are proximate to the firstliquid crystal alignment layer have a first pretilt angle from about 2degrees to about 15 degrees and a first azimuthal direction; said secondsubstrate having a second liquid crystal alignment layer proximate tosaid liquid crystal fluid, wherein molecules of said liquid crystalfluid that are proximate to the second liquid crystal alignment layerhave a second pretilt angle from about 2 degrees to about 15 degrees anda second azimuthal direction, the second azimuthal direction beingsubstantially perpendicular to the first azimuthal direction; wherein:when an electric field is applied between said first and secondsubstrates, a substantial number of the molecules of said liquid crystalfluid increase their tilt angles, and when substantially no electricfield is applied between said first and second substrates, a substantialnumber of the molecules of said liquid crystal fluid have theirazimuthal direction substantially perpendicular to said first and secondsubstrates; whereby when the electric field is applied between saidfirst and second substrates, said liquid crystal fluid changes linearpolarized incident light at said first substrate to approximatelycircularly polarized incident light when at said second substrate,wherein said second substrate reflects back the approximately circularlypolarized incident light as approximately circularly polarized light ofopposite handedness, wherein said liquid crystal fluid changes theapproximately circularly polarized reflected light to approximatelylinear polarized reflected light when at said first substrate such thatthe polarization directions of the linearly polarized incident light andthe linearly polarized reflected light at said first substrate areapproximately perpendicular, and whereby when substantially no electricfield is applied between said first and second substrates, said liquidcrystal fluid does not substantially change the polarization of lightpassing through the liquid crystal fluid and the polarization directionsof the linearly polarized incident light and the linearly polarizedreflected light at said first substrate are approximately parallel. 15.The reflective liquid crystal display of claim 14, wherein the electricfield is applied between said first and second substrates for aplurality of first times and substantially no electric field is appliedfor a plurality of second times.
 16. The reflective liquid crystaldisplay of claim 15, wherein the first and second times are varied toproduce shades of gray.
 17. The reflective liquid crystal display ofclaim 15, wherein the first and second times are varied to produce fieldsequential colors.
 18. A method for a reflective liquid crystal display,said method comprising the steps of: providing a first substrate that issubstantially transparent; providing a second substrate that issubstantially reflective and substantially parallel with said firstsubstrate; and providing a liquid crystal fluid having a birefringence(Δn) and a negative dielectric anisotropy between said first and secondsubstrates; providing a first liquid crystal alignment layer on saidfirst substrate, said first liquid crystal alignment layer beingproximate to said liquid crystal fluid, wherein molecules of said liquidcrystal fluid that are proximate to the first liquid crystal alignmentlayer have a first pretilt angle from about 2 degrees to about 15degrees and a first azimuthal direction; providing a second liquidcrystal alignment layer on said second substrate, said second liquidcrystal alignment layer being proximate to said liquid crystal fluid,wherein molecules of said liquid crystal fluid that are proximate to thesecond liquid crystal alignment layer have a second pretilt angle fromabout 2 degrees to about 15 degrees and a second azimuthal direction,the second azimuthal direction being substantially perpendicular to thefirst azimuthal direction; such that when substantially no electricfield is applied between said first and second substrates, said liquidcrystal fluid does not change the polarization state of light passingtherethrough and the polarization directions of the linearly polarizedincident light and the linearly polarized reflected light at said firstsubstrate are substantially parallel, when applying an electric field ofan optimum value between said first and second substrates, said liquidcrystal fluid changes linear polarized incident light at said firstsubstrate to approximately circularly polarized incident light when atsaid second substrate, wherein said second substrate reflects back theapproximately circularly polarized incident light as approximatelycircularly polarized light of opposite handedness, wherein said liquidcrystal fluid changes the approximately circularly polarized reflectedlight to approximately linear polarized reflected light when at saidfirst substrate such that the polarization directions of the linearlypolarized incident light and the linearly polarized reflected light atsaid first substrate are approximately perpendicular, and when applyingan electric field less than the optimum value between said first andsecond substrates, said liquid crystal fluid changes the polarizationstate of the incident linearly polarized light to elliptically polarizedlight upon passing through said liquid crystal fluid and being reflectedback from said second substrate.
 19. The method of claim 18, wherein thepolarization direction of the linearly polarized incident light isapproximately parallel with the first azimuthal direction.
 20. Themethod of claim 18, wherein the polarization direction of the linearlypolarized incident light is approximately perpendicular with the firstazimuthal direction.
 21. The method of claim 18, further comprising thestep of varying the electric field between said first and secondsubstrates so as to produce shades of gray by causing the polarizationstate of the incident light at said second substrate to vary fromapproximately linear polarization to elliptical polarization, and whenthere is substantially no electric field the polarization of theincident light at said second substrate is approximately linear.
 22. Themethod of claim 18, wherein a distance (d) between inside faces of saidfirst and second substrates is about 3.5 micrometers.
 23. The method ofclaim 18, wherein a distance (d) between inside faces of said first andsecond substrates is from about 3.3 micrometers to about 3.7micrometers.
 24. The method of claim 18, wherein said liquid crystalfluid has a birefringence (Δn) of about 0.0830 at about 45 degreesCelsius.
 25. The method of claim 18, wherein said liquid crystal fluidhas a birefringence (Δn) from about 0.0777 to about 0.0996.
 26. Themethod of claim 18, wherein the birefringence An times d is greater thanλ/4 when an electric field having an optimum value is applied betweensaid first and second substrates, where d is the distance between insidefaces of said first and second substrates and λ is the wavelength oflight.
 27. The method of claim 18, wherein the molecules of said liquidcrystal fluid have a tilt angle (θ) of from about 5 degrees to about 15degrees when substantially no electric field is applied between saidfirst and second substrates.
 28. The method of claim 18, wherein themolecules of said liquid crystal fluid have a tilt angle (θ) of fromabout 2 degrees to about 15 degrees when substantially no electric fieldis applied between said first and second substrates.
 29. The method ofclaim 18, wherein an azimuthal angle (Φ) of the molecules of said liquidcrystal fluid varies from about 0 degrees at said first substrate toabout 90 degrees at said second substrate when substantially no electricfield is applied between said first and second substrates.
 30. Themethod of claim 18, wherein an azimuthal angle (Φ) of the molecules ofsaid liquid crystal fluid varies from about 0 degrees at said secondsubstrate to about 90 degrees at said first substrate when substantiallyno electric field is applied between said first and second substrates.31. A method for a reflective liquid crystal display, said methodcomprising the steps of: providing a first substrate that issubstantially transparent; providing a second substrate that issubstantially reflective and substantially parallel with said firstsubstrate; and providing a liquid crystal fluid having a birefringence(Δn) and a negative dielectric anisotropy, wherein said liquid crystalfluid is between said first and second substrates; providing a firstliquid crystal alignment layer on said first substrate, said firstliquid crystal alignment layer being proximate to said liquid crystalfluid, wherein molecules of said liquid crystal fluid that are proximateto the first liquid crystal alignment layer have a first pretilt anglefrom about 2 degrees to about 15 degrees and a first azimuthaldirection; providing a second liquid crystal alignment layer on saidsecond substrate, said second liquid crystal alignment layer beingproximate to said liquid crystal fluid, wherein molecules of said liquidcrystal fluid that are proximate to the second liquid crystal alignmentlayer have a second pretilt angle from about 2 degrees to about 15degrees and a second azimuthal direction, the second azimuthal directionbeing substantially perpendicular to the first azimuthal direction;wherein: when applying substantially no electric field between saidfirst and second substrates, a substantial number of the molecules ofsaid liquid crystal fluid have their azimuthal direction substantiallyperpendicular to said first and second substrates; and when applying anelectric field between said first and second substrates, a substantialnumber of the molecules of said liquid crystal fluid increase their tiltangles; whereby when applying the electric field between said first andsecond substrates, said liquid crystal fluid changes linear polarizedincident light at said first substrate to approximately circularlypolarized incident light when at said second substrate, wherein saidsecond substrate reflects back the approximately circularly polarizedincident light as approximately circularly polarized light of oppositehandedness, wherein said liquid crystal fluid changes the approximatelycircularly polarized reflected light to approximately linear polarizedreflected light when at said first substrate such that the polarizationdirections of the linearly polarized incident light and the linearlypolarized reflected light at said first substrate are approximatelyperpendicular, and whereby when applying substantially no electric fieldbetween said first and second substrates, said liquid crystal fluid doesnot substantially change the polarization state of light passing throughsaid liquid crystal fluid such that the polarization directions of thelinearly polarized incident light and the linearly polarized reflectedlight at said first substrate are approximately parallel.
 32. Areflective liquid crystal display assembly comprising: a firstsubstantially transparent substrate; a second substantially reflectivesubstrate located substantially parallel to said first substrate; aliquid crystal fluid having a birefringence (Δn) and a negativedielectric anisotropy, wherein said liquid crystal fluid is between saidfirst and second substrates; first and second liquid crystal alignmentlayers on said first and second substrates, respectively, whereinmolecules of said fluid proximate the first and second liquid crystalalignment layers have finite pretilt angles and are oriented in firstand second azimuthal directions, respectively; wherein the assembly isconfigured such that (i) when substantially no electric field is appliedbetween the substrates, a substantial number of fluid molecules areoriented substantially perpendicular to said substrates, (ii) when anelectrical field of optimum value is applied between the substrates,tilt angles of a substantial number of fluid molecules increase, and(iii) when a less than optimum electric field is applied between saidsubstrates, a substantial number of fluid molecules are oriented atintermediate tilt angles.